RTP Control Protocol (RTCP) Extended Report (XR)

for Post-Repair Loss Count Metrics

Abstract

This document defines an RTP Control Protocol (RTCP) Extended Report
(XR) block that allows reporting of a post-repair loss count metric
for a range of RTP applications. In addition, another metric,
repaired loss count, is also introduced in this report block for
calculating the pre-repair loss count when needed, so that the RTP
sender or a third-party entity is able to evaluate the effectiveness
of the repair methods used by the system.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7509.

Copyright Notice

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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

1. Introduction

RTCP Sender Reports (SRs) / Receiver Reports (RRs) [RFC3550] contain
some rough statistics about the data received from the particular
source indicated in that block. One of them is the cumulative number
of packets lost, which is called the pre-repair loss metric in this
document. This metric conveys information regarding the total number
of RTP data packets that have been lost since the beginning of the
RTP session.

However, this metric is measured on the media stream before any loss-
repair mechanism, e.g., retransmission [RFC4588] or Forward Error
Correction (FEC) [RFC5109], is applied. Using a repair mechanism
usually results in recovering some or all of the lost packets. The
recovery process does not reduce the values reported by the two loss
metrics in RTCP RR [RFC3550] -- namely, the fraction lost and the
cumulative loss. Hence, the sending endpoint cannot infer the
performance of the repair mechanism based on the aforementioned
metrics in [RFC3550].

Consequently, [RFC5725] specifies a post-repair loss Run-Length
Encoding (RLE) XR report block to address this issue. The sending
endpoint is able to infer which packets were repaired from the RLE
report block, but the reporting overhead for the packet-by-packet
report block is higher compared to other report blocks.

When applications use multiple XR blocks, the endpoints may require
more concise reporting to save bandwidth. This document defines a
new XR block type to augment those defined in [RFC3611] and
complement the report block defined in [RFC5725] for use in a range
of RTP applications. This new block type reports the post-repair
loss count metric, which records the number of primary source RTP
packets that are still lost after applying one or more loss-repair
mechanisms. In addition, another metric, repaired loss count, is
also introduced in this report block for calculating the pre-repair
loss count during this range, so that the RTP sender or a third-party
entity is able to evaluate the effectiveness of the repair methods
used by the system. The metrics defined in this document are packet
level rather than slice/picture level; this means the partial
recovery of a packet will not be regarded as a repaired packet.

The metrics defined in this document belong to the class of
transport-related metrics defined in [RFC6792] and are specified in
accordance with the guidelines in [RFC6390] and [RFC6792]. These
metrics are applicable to any RTP application, especially those that
use loss-repair mechanisms.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].

primary source RTP packet: The original RTP packet sent from the RTP

sender for the first time. A lost primary source RTP packet may
be repaired by some other RTP packets used in repair mechanisms
like FEC or retransmission.

3. Post-Repair Loss Count Metrics Report Block

This block reports the number of packets lost after applying repair
mechanisms (e.g., FEC). It complements the RTCP XR metrics defined
in [RFC5725]. As noted in [RFC5725], ambiguity may occur when
comparing this metric with a pre-repair loss metric reported in an
RTCP SR/RR, i.e., some packets were not repaired in the current RTCP
interval, but they may be repaired later. Therefore, this block uses
a begin sequence number and an end sequence number to explicitly
indicate the actual sequence number range reported by this RTCP XR.
Accordingly, only packets that have no further chance of being
repaired and that have been repaired are included in this report
block.

3.1. Report Block Structure

The Post-Repair Loss Count Metrics Report Block has the following
format:

The first sequence number that this block reports on. It can
remain fixed when calculating metrics over several RTCP reporting
intervals.

end_seq:

bits

The last sequence number that this block reports on plus one.

Post-repair loss count: 16 bits

Total number of packets finally lost after applying one or more
loss-repair methods, e.g., FEC and/or retransmission, during the
actual sequence number range indicated by begin_seq and end_seq.
This metric MUST NOT count the lost packets for which repair might
still be possible. Note that this metric MUST measure only
primary source RTP packets.

Repaired loss count: 16 bits

Total number of packets fully repaired after applying one or more
loss-repair methods, e.g., FEC and/or retransmission, during the
actual sequence number range indicated by begin_seq and end_seq.
Note that this metric MUST measure only primary source RTP
packets.

3.2 Example Usage

The metrics defined in this report block are all measured at the RTP
receiver. However, the receiving endpoint can report the metrics in
two different ways:

Cumulative report

In this case, implementations may set begin_seq to the first packet
in the RTP session, and it will remain fixed across all reports.
Hence, the "Post-repair loss count" and "Repaired loss count",
respectively, will correspond to "Cumulative post-repair loss count"
and "Cumulative repaired loss count" in this case. These cumulative
metrics when combined with the cumulative loss metrics reported in an
RTCP RR (pre-repair) assist in calculating the "Still-to-be-repaired
lost packets":

Some implementations may align the begin_seq and end_seq number with
the highest sequence numbers of consecutive RTCP RRs (RTCP interval).
This is NOT RECOMMENDED as packets that are not yet repaired in this
current RTCP interval and may be repaired in the subsequent intervals
will not be reported. An interval report is illustrated in the
following example:

Interval A:

The extended highest sequence number received in RTCP
RR is 20. Begin_seq is 10 and end_seq is 20.

Interval B:

The extended highest sequence number received in RTCP
RR is 30. Begin_seq is 20 and end_seq is 30.

If packets 17 and 19 are lost and not yet repaired in interval A and
subsequently repaired in interval B, they will not be reported
because their sequence numbers do not belong in interval B.
Therefore, if implementations want these packets to be reported as
repaired, they MUST NOT align the begin_seq and end_seq to the RTCP
intervals.

Alternatively, implementations may choose the begin_seq and end_seq
numbers that cover several RTCP intervals. Additionally, the
reported range of sequence numbers may overlap with the previous
report blocks, so that the packets that were not yet repaired in one
interval, but were subsequently repaired or deemed unrepairable, were
reported in subsequent intervals.

In this case, the "Cumulative number of packets lost" cannot be
easily compared with the post-repair metrics. However, the sending
endpoint can calculate the efficiency of the error resilience
algorithm using the post-repair and repaired loss count,
respectively.

4. SDP Signaling

[RFC3611] defines the use of SDP (Session Description Protocol) for
signaling the use of RTCP XR blocks. However, XR blocks MAY be used
without prior signaling (see Section 5 of [RFC3611]).

4.1. SDP rtcp-xr-attrib Attribute Extension

This session augments the SDP attribute "rtcp-xr" defined in Section
5.1 of [RFC3611] by providing an additional value of "xr-format" to
signal the use of the report block defined in this document. The
ABNF [RFC5234] syntax is as follows.

xr-format =/ xr-prlr-block
xr-prlr-block = "post-repair-loss-count"

4.2. Offer/Answer Usage

When SDP is used in offer/answer context, the SDP Offer/Answer usage
defined in [RFC3611] for the unilateral "rtcp-xr" attribute
parameters applies. For detailed usage of Offer/Answer for
unilateral parameters, refer to Section 5.2 of [RFC3611].

5. Security Considerations

This proposed RTCP XR block introduces no new security considerations
beyond those described in [RFC3611]. This block does not provide
per-packet statistics, so the risk to confidentiality documented in
Section 7, paragraph 3 of [RFC3611] does not apply.

An attacker may put incorrect information in the Post-Repair Loss
Count reports, which will affect the performance of loss-repair
mechanisms. Implementers should consider the guidance in [RFC7202]
for using appropriate security mechanisms, i.e., where security is a
concern, the implementation should apply encryption and
authentication to the report block. For example, this can be
achieved by using the AVPF profile together with the Secure RTP
profile as defined in [RFC3711]; an appropriate combination of the
two profiles (an "SAVPF") is specified in [RFC5124]. However, other
mechanisms also exist (documented in [RFC7201]) and might be more
suitable.

6. IANA Considerations

New block types for RTCP XR are subject to IANA registration. For
general guidelines on IANA considerations for RTCP XR, refer to
[RFC3611].

Metric Description: Total number of RTP packets still lost
after loss-repair methods are applied.

Method of Measurement or Calculation: See the "Post-repair
loss count" definition in Section 3.1. It is directly
measured and must be measured for the primary source RTP
packets with no further chance of repair.

Units of Measurement: This metric is expressed as a 16-bit
unsigned integer value giving the number of RTP packets.

Measurement Point(s) with Potential Measurement Domain: It is
measured at the receiving end of the RTP stream.

Measurement Timing: This metric relies on the sequence number
interval to determine measurement timing. See the Cumulative
and Interval reports defined in Section 3.2.

Use and Applications: These metrics are applicable to any RTP
application, especially those that use loss-repair mechanisms.
See Section 1 for details.